Pressure sensor devices have been widely employed in the prior art for various applications and in many different environments. The prior art is replete with a number of patents for pressure sensor devices that employ semiconductor transducer elements. Such transducers typically utilize piezoresistive elements as the force responsive members. By utilizing a semiconductor transducer, the prior art is enabled to provide a reliable pressure sensor that is relatively inexpensive to fabricate and exhibits a high degree of reliability.
As one can ascertain, prior art transducers have been employed in many different environments that subject the transducers to stringent operating conditions such as high temperatures and high pressure. For example, such transducers have been used in the automotive field to monitor engine pressure, coolant pressure and so on. They are used in the aircraft field to measure aerodynamic pressures and in many other varied fields where fluid pressure is a concern. Apart from the high operating temperatures and pressures, the transducers have been subjected to various pollutants and gasses which are present in such environments.
In a sensor assembly that utilizes a semiconductor pressure transducer, typically a piezoresistive element is mounted on, or diffused within, a diaphragm structure. The diaphragm structure is then mechanically, pneumatically or hydraulically coupled to the gas or fluid being monitored so that changes in fluid pressure can cause the diaphragm structure to deform. The deformation of the diaphragm structure is transferred to the piezoresistive element. The resistance of the piezoresistive element varies with the degree of deformation of the diaphragm structure. As a result, the resistance of the piezoresistive element is indicative of the fluid pressure being monitored.
The diaphragm structure is typically fabricated from a semiconductor material, ceramic or glass and is relatively fragile. As one can ascertain, the diaphragm structure is subjected to large changes in pressure and therefore deflect according to the magnitude of such changes in pressure. As indicated, typical prior art diaphragm structures are relatively fragile and are commonly supported by a conventional housing or annular structure. For examples of typical support mechanisms for such diaphragm, reference is made to U.S. Pat. No. 4,216,404 entitled HOUSING AND LEAD ARRANGEMENTS FOR ELECTROMECHANICAL TRANSDUCERS, issued Aug. 5, 1980 to A. D. Kurtz and Joseph R. Mallon, Jr. and assigned to Kulite Semiconductor Products, Inc., the assignee herein. Reference is also made to U.S. Pat. No. 3,654,579 entitled ELECTROMECHANICAL TRANSDUCERS AND HOUSINGS, issued on Apr. 4, 1972 to A. D. Kurtz, Joseph R. Mallon, Jr. and Charles Gravel and assigned to the assignee herein, and U.S. Pat. No. 4,764,747 entitled GLASS HEADER STRUCTURE FOR A SEMICONDUCTOR PRESSURE TRANSDUCER, issued Aug. 16, 1988 to A. D. Kurtz, Joseph R. Mallon and Timothy A. Nun and assigned to the assignee herein.
It has been a major object of the prior art to provide a reliable transducer structure that is both easy to manufacture and inexpensive. A major problem in providing such a transducer relates to the diaphragm structure upon which the piezoresistive elements rest. In forming many diaphragm structures within transducers, the piezoresistive elements are affixed to, or diffused within, a semiconductor or glass substrate. The material of the substrate is then etched away from below the piezoresistive elements so that the substrate is at its thinnest just below the piezoresistive elements. The etched regions operate as stress concentrating areas within the diaphragm support structure and enable the piezoresistive elements to provide a relatively large output upon application of a deflecting force to the diaphragm structure.
When piezoresistive elements are joined to the diaphragm structure, they are conventionally arranged in a Wheatstone bridge circuit having four interconnected piezoresistive elements. As such, it is very important that the thickness of the diaphragm structure be identical below each element. If the thickness of the diaphragm structure varies, the amount of deflection will be unbalance, thereby causing the overall transducer not to be reliable. It is also the function of the diaphragm structure to isolate the piezoresistive elements from the influence of external stresses not caused by a change in the fluid pressure being monitored. Such external stresses may be of mechanical origin, but are commonly caused by a mismatch in the rates of thermal expansion between the material of the piezoresistive elements and the material of the below lying diaphragm structure. If the material of the piezoresistive elements does not match that of the diaphragm structure, the difference in thermal expansion across a given temperature range causes the piezoresistive elements to be falsely stressed, thereby detracting from the reliability of the overall transducer.
It is, therefore, an object of the present invention to provide a diaphragm structure for a semiconductor piezoresistive element that can be etched to exact tolerances so as to evenly support the piezoresistive elements and provide a coefficient of thermal expansion substantially equivalent to that of the piezoresistive elements.
It is a further object of the present invention to provide such a diaphragm structure that is both inexpensive and easy to manufacture and provides superior performance characteristics over comparable prior art structures.